1. Field of the Invention
The field of the present invention is techniques for fabricating microresonators, particularly ultra-high Q optical microresonators which are detachable from the structures upon which they are initially fabricated to be freely positioned within a photonics circuit.
2. Background
Optical microring resonators (OMRs) are versatile elements for designing integrated photonic circuits, and can be used as building blocks for many optical signal processing devices and systems. OMRs can be employed in many different types of photonic circuits, including multipole add-drop filters, multistage dispersion compensators and delay lines, electro-optic modulators, lasers, nonlinear optical elements, and sensor applications. Currently the OMRs employed in most photonic devices are fabricated using traditional monolithic techniques using various material systems such as polymers, III-V semiconductors, Hydex™, SiN, and Si, among others. The quality factor of these OMRs is below 106 and is mainly limited by the surface roughness of the sidewalls, which is dictated by a fundamental limitation for currently available etching techniques. Moreover the monolithic fabrication of these devices prevents post fabrication position tuning, thus resulting in device performance that is limited by the accuracy of the lithographical techniques. In the design of multipole filter and dispersion compensating elements, where the device performance is extremely sensitive to coupling factors, post fabrication position tuning can play a crucial role in achieving the desired response. In fact, some form of post fabrication tuning is currently used for final tuning of most multipole microwave and RF filters.
Ultra-high-Q (UH-Q) microtoroid silica resonators represent a distinct class of OMRs with Q factors in excess of 100 million. This exceptional quality factor is a result of employing a special fabrication process that generates surface tension induced smoothness on the resonator sidewalls. This chip-based and relatively simple fabrication process allows fast production of UH-Q microtoroids with relatively high yield. Unfortunately this special process limits the range of possible integration choices for these resonators. The XeF2 dry etching and the CO2 laser reflow process can damage the microstructures and the devices built in the vicinity of the microtoroid. Also the diameter of the silica microdisk shrinks down as it reflows to its final toroidal shape. After fabrication, microtoroids are physically perched atop a silicon pillar so those photonic devices that rely on coupling of these resonators to an integrated waveguide or their mutual coupling cannot be realized.
At the same time, the low intrinsic loss of UH-Q microtoroidal resonators makes them very attractive for applications where low loss, large circulating power, narrow bandwidth, and large dispersion are required. For example, by employing UH-Q microtoroids in multi-ring filters, bandwidths and insertion losses are possible that can outperform existing resonant optical filters. Such filters would be unprecedented in optics and have immediate applications for RF photonic signal processing and DWDM optical communication systems. Taper-coupled UH-Q microtoroids have also been shown capable of reaching exceedingly high-power transfer efficiencies in four port couplers. In another example, high quality factor has a crucial role in the operation of optical ring resonator based biosensors that have been the subject of research in recent years, and silica microtoroid sensors have been shown capable of boosting the sensitivity of the resonant optical sensors by many orders of magnitude. In yet another example, integration of microtoroidal resonators with existing microfluidic and biophotonic devices can result in functionalities that have not been realized using existing monolithic microresonators. Moreover it has been shown that microtoroidal resonators may be used as ultra-low threshold laser sources having very narrow linewidths—Raman, Erbium and Ytterbium lasers have been already demonstrated. Such lasers can considerably improve the functionality of existing photonic systems.
Furthermore, the combination of the low cavity losses, small mode volume, and relative ease of fabrication makes microtoroidal resonators promising candidates for cavity QED (cavity quantum electrodynamics) experiments. This use has manifested by the recent demonstration of strong coupling between individual cesium atoms and the field of a high-Q optical mode in a microtoroid. Generally, there is great interest in finding ways to realize on-chip, strongly coupled systems. In addition, the ability of building a network of coupled microtoroids creates a powerful platform for realizing new cavity QED experiments. The only substitute for silica microtoroid cavities are UH-Q crystalline microcavities, which have with optical Q factors above 1010. These resonators are fabricated using computer controlled mechanical grinding techniques and currently they can have diameters as small as 200 μm. However, between the complex fabrication techniques and the fact that these resonators are usually embedded in a crystalline rod, miniaturization and integration of crystalline resonators has not yet been realized. Microtoroids, on the other hand, already have the benefit of size on the order of tens of microns, leaving integration as one of the remaining challenges.